1. Field of the Invention
The present invention relates to a radiation image radiographing apparatus, in particular, a radiation image radiographing apparatus capable of radiographing a phase contrast image.
2. Related Art
Generally, a radiation image radiographing apparatus using a mechanism of transmitting radiation through a substance is widely used for a medical image diagnosis, a non-destructive inspection and the like. In particular, in a radiation image radiographing apparatus for mammography, which is used for radiographing a specific region of body, radiography of a subject fixed on a subject platform which is combined with a radiation image information detecting member has been performed. However, since the subject is radiographed at substantially full scale with this method, contrast of the radiographed image is insufficient. Thereby, the radiation image radiographing apparatus is not capable of radiographing an image clear enough to provide fine structure of the specific region of a body for interpretation.
Accordingly, a method of obtaining a phase contrast image by use of a radiation tube (a small focus radiation source having a focus size within the range from 30 μm to 300 μm), which is used in general medical institutions, is known (for example, refer to Japanese Patent Application Publication (Unexamined) No. Tokukai 2001-238871). With this method, it is possible to render a border with higher contrast, and thereby obtain a clearer and finer image than the normal absorption contrast image. However, in order to obtain such a phase contrast image, it is necessary to have predetermined distance between the subject and the radiation image information detecting member. Further, in view of reducing a subject's burden and considering cost to spend on equipment in a medical scene, ideally, the radiographing apparatus should be capable of performing radiography in both “phase contrast image radiography mode” for radiographing a phase contrast image and “absorption contrast image radiography mode” for radiographing only an absorption contrast image.
Accordingly, in an earlier art, a structure where a supporting member for supporting the radiation image information detecting member is placed and capable of moving vertically along a rail, and thereby it is possible to change the distance between the subject and the radiation image information detecting member, is used. Therefore, in the earlier art, the radiographing apparatus is capable of radiographing both a normal absorption contrast image and a phase contrast image.
However, with the method in the earlier art, since the supporting platform is capable of moving its location arbitrarily, a magnifying rate of the image also changes countlessly. Therefore, it is necessary for an operator to adjust the magnifying rate for radiographing. Consequently, when it is necessary to obtain images with the same magnifying rate, even though calibration of the magnifying rate is provided, it is not easy to make a fine adjustment.
Further, since it is also necessary to have a location recognition device as a means to recognize a location for obtaining information of distance between a radiation source and the subject platform (with which the subject is in contact) and the distance between the subject platform and the radiation image information detecting member, control radiation dose based on the information and the like, the control becomes more complicated.
Furthermore, since it is necessary to have such a complicated control means as mentioned above, it causes an increase on cost of the apparatus.